The present invention relates to a process for manufacturing a solid-state image pick-up device. More particularly, the invention relates to a process for manufacturing a charge transfer section, used in a solid-state image pick-up device, for transferring charges from vertical transfer registers to a horizontal transfer register.
As a solid-state image pick-up device, on the whole, becomes smaller, the dimensions of the individual elements of the device accordingly decrease. As an example of the downsizing of the elements of the device, the channel width of a charge transfer section for transferring signal charges from vertical transfer registers to a horizontal transfer register (hereinafter referred to as "the V-H transfer section") is decreased.
FIG. 8 is an elevational view of a conventional V-H transfer section. FIG. 9 is a cross sectional view along line Y-Y' of FIG. 8. Referring to FIGS. 8 and 9, a vertical transfer register 81 is comprised of channels 83 and electrodes 84-1 and 84-4 (only the electrode 84-4 is shown in FIG. 8) that are driven in, for example, four phases. The channels 83 are formed by ion-implanting N+-type impurities into the upper top surface of a silicon substrate 82. The four-phase electrodes 84-1 to 84-4 are disposed above the channels 83 and are sequentially arranged in the transfer direction. On the other hand, a horizontal transfer register 85 includes a channel 86 and electrode pairs 87-1 and 87-2, driven in, for example, two phases, each pair forming a storage electrode ST and a transfer electrode TR. The channel 86 is formed by ion-implanting N+-type impurities into the obverse surface of the silicon substrate 82. On the other hand, the electrode pairs 87-1 and 87-2 are disposed above the channel 86 and are sequentially arranged in the transfer direction. In each of the electrode pairs 87-1 and 87-2, the storage electrode ST (indicated by the one-dot broken line in FIG. 8) is formed of a first polysilicon layer, while the transfer electrode TR (designated by the two-dot broken line in FIG. 8) is formed of a second polysilicon layer.
Further, the transfer electrode TR of the first phase (.phi.H1) electrode pair 87-1 extends until the fourth-phase (.phi.V4) electrode 84-4 along the channel 83 of the vertical transfer register 81. The extending portion TR' of the transfer electrode TR projecting from the channel 86, together with the channel 83 positioned under the portion TR', form a V-H transfer section 88 for transferring signal charges from the vertical transfer register 81 to the horizontal transfer register 85.
In the V-H transfer section 88 constructed as described above, the channel width is determined by the manufacturing precision of the channel 83 and the transfer electrode TR of the first-phase (.phi.H1) electrode pair 87-1 of the horizontal transfer register 85. More specifically, in consideration of the matching precision of the channel 83 to the extending portion TR' of the transfer electrode TR of the first-phase (.phi.H1) electrode pair 87-1 during manufacturing of the V-H transfer section 88, the channel 83 should be formed within the edges of the transfer electrode TR, away from each edge across a predetermined distance (margin) A so that it does not project from the extending portion TR'.
As described above, in the conventional solid-state image pick-up device, the channel width of the V-H transfer section 88 depends on the manufacturing precision of the channel 83 and the transfer electrode TR of the first-phase electrode pair 87-1 of the horizontal transfer register 85. Thus, considering manufacturing precision, the channel width of the V-H transfer section 88 cannot be increased to a great degree. Under this restriction, if the individual elements of the solid-state image pick-up device are decreased along with the downsizing of the overall device, the channel width of the V-H transfer section 88 inevitably should be made smaller. However, an excessively smaller channel width of the V-H transfer section 88 degrades the transfer efficiency of signal charges from the vertical transfer register 81 to the horizontal transfer register 85. This imposes a restriction of the downsizing of the solid-state image pick-up device.